Organic electroluminescent (EL) devices or organic light-emitting devices (OLEDs) are electronic devices that emit light in response to an applied potential. The structure of an OLED comprises, in sequence, an anode, an organic EL medium, and a cathode. The organic EL medium disposed between the anode and the cathode is commonly comprised of an organic hole-transporting layer (HTL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the ETL near the interface of HTL/ETL. Tang et al. [“Organic Electroluminescent Diodes”, Applied Physics Letters, 51, 913 (1987), and commonly assigned U.S. Pat. No. 4,769,292] demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures have been disclosed. For example, there are three-layer OLEDs that contain an organic light-emitting layer (LEL) between the HTL and the ETL, such as that disclosed by Adachi et al., “Electroluminescence in Organic Films with Three-Layer Structure”, Japanese Journal of Applied Physics, 27, L269 (1988), and by Tang et al., “Electroluminescence of Doped Organic Thin Films”, Journal of Applied Physics, 65, 3610 (1989). The LEL commonly consists of a host material doped with a guest material. Wherein the layer structures are denoted as HTL/LEL/ETL, Further, there are other multilayer OLEDs that contain a hole-injecting layer (HIL), and/or an electron-injecting layer (EIL), and/or a hole-blocking layer, and/or an electron-blocking layer in the devices. These structures have further resulted in improved device performance.
Moreover, in order to further improve the performance of the OLEDs, a new kind of OLED structure called stacked OLED (or cascaded OLED), which is fabricated by stacking several individual OLEDs vertically, has also been proposed. Forrest et al. in U.S. Pat. No. 5,703,436 and Burrows et al. in U.S. Pat. No. 6,274,980 disclosed their stacked OLEDs. In their inventions, the stacked OLEDs are fabricated by vertically stacking several OLEDs, each independently emitting light of a different color or of the same color. They believe that by using their stacked OLED structure, full color emission devices with higher integrated density in the display could be made. However, each OLED unit in their devices needs a separate power source. In an alternative design, Tanaka et al. in U.S. Pat. No. 6,107,734 and Jones et al. in U.S. Pat. No. 6,337,492 proposed a stacked OLED structure by vertically stacking several OLEDs without individually addressing each OLED unit in the stack. Tanaka et al. indicated that their stacked structure could increase the luminance output or operational lifetime.
The aforementioned stacked OLEDs use metals, metal alloys, or other inorganic compounds as intermediate electrodes, with the electrical resistivity lower than 0.1 Ω-cm, to connect each individual OLED unit in the stacked OLED. These device architectures present serious fabrication difficulties. First, the fabrication of the intermediate electrodes, such as an In—Zn—O oxide film or an In—Sn—O oxide film, is an ion-sputtering process. This method will cause damage on organic surfaces [Liao et al., “Ion-beam-induced surface damages on tris-(8-hydroxyquinoline) aluminum”, Applied Physics Letters, 75, 1619 (1999)]. Second, due to their thickness limitation, these electrodes cannot be used as spacers to adjust the light reflecting length in the device for improving light extraction. Third, if an intermediate electrode is a discontinuous layer, its carrier injecting ability will be inferior, rendering poor EL performance. If the intermediate electrode is a continuous thicker layer, it will have high lateral conductivity resulting in severe pixel cross-talk, and will have low optical transparency resulting in reduced light extraction. (Pixel cross-talk means that the pixels adjacent to a lit pixel emit unwanted light due to high lateral conductivity). In the prior art, either a shadow mask is used for pixelization of each intermediate electrode or an insulating layer is deposited to define each pixel on the substrate at an early step in the OLED fabrication. Both of the methods would be complicated, resulting in low production yield.